1. Field of the Invention
The present invention relates to an apparatus for and a method of capturing a radiation image by applying a radiation emitted from a radiation source to a breast and detecting the radiation that has passed through the breast with a radiation detector.
2. Description of the Related Art
In the medical field, for example, it has been customary to apply a radiation to a subject and detect the radiation that has passed through the subject with a radiation detector, thereby capturing a radiation image of the subject.
One known radiation detector comprises a solid-state detector including a matrix of charge collecting electrodes formed on an insulating substrate and a radiation conductor disposed on the charge collecting electrodes for generating electric charges depending on the radiation that is applied to the radiation detector. The electric charges generated by the radiation conductor and representing a radiation image are collected by the charge collecting electrodes and temporarily stored in an electric storage unit. The collected electric charges are converted into an electric signal, which is output from the radiation detector. Other known radiation detectors include a radiation detector comprising a charge-coupled device (CCD) and a radiation detector comprising a combination of amorphous silicon and a scintillator. Furthermore, a stimulable phosphor panel which, when exposed to an applied radiation (X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet radiation, or the like), stores part of the energy of the radiation, and, when subsequently exposed to applied stimulating light such as laser beam, visible light, or the like, emits photo-stimulated luminescence in proportion to the stored energy of the radiation, may also be used as a radiation detector.
In order to obtain a high-quality radiation image captured by a radiation detector, it is necessary to set an appropriate irradiation dose depending on the radiation transmittance of the region to be imaged, of the subject. If the region to be imaged is a breast, then since the radiation transmittance greatly differs depending on the thickness of the breast and the density of the mammary gland, it is highly important to adjust the required irradiation dose depending on the state of the breast and control the radiation to prevent the breast from being irradiated with an excessive amount of radiation.
Japanese Laid-Open Patent Publication No. 7-153592 and Japanese Laid-Open Patent Publication No. 2003-115399 disclose a process of performing a pre-exposure mode to apply a low dose of radiation to a subject, detect the dose of the radiation that has passed through the subject with a radiation detector, and calculate a irradiation dose required to capture a radiation image of the subject and thereafter performing a main exposure mode to apply a radiation based on the calculated irradiation dose to the subject to capture a radiation image of the subject.
However, inasmuch as the radiation that reaches the radiation detector greatly differs depending on the thickness of the breast and the density of the mammary gland, if the irradiation dose in the pre-exposure mode is set to a constant level as disclosed in Japanese Laid-Open Patent Publication No. 7-153592 and Japanese Laid-Open Patent Publication No. 2003-115399, a radiation having a dose which needs to be detected may not be applied to the radiation detector or an excessive radiation may be applied to the radiation detector, so that the radiation detector may fail to detect the irradiation dose highly accurately. If an excessive radiation greater than the irradiation dose required to capture a radiation image is applied to the breast in the pre-exposure mode, then an unnecessary radiation is applied to the breast, tending to make the main exposure mode impossible to perform.